Readback waveform oversampling method and apparatus

ABSTRACT

A read channel is configured to obtain an analog readback waveform from a magnetic recording medium of a disk drive at a sampling rate of one sample per one written bit. A buffer is coupled the read channel. Circuitry is configured to inject a plurality of different phase offsets into the read channel for each of a plurality of revolutions of the medium. The circuitry is also configured to store, in a buffer, an amplitude of the readback waveform for each of the different phase offsets. The circuitry is further configured to generate an oversampled readback waveform using the amplitudes stored in the buffer.

RELATED APPLICATIONS

This application claims the benefit of Provisional Patent ApplicationSer. No. 62/674,172 filed on May 21, 2018, which is hereby incorporatedherein by reference in its entirety.

SUMMARY

Various embodiments are directed to a method comprising obtaining, via aread channel of a disk drive, a readback waveform from a magneticrecording medium at a sampling rate of one sample per one written bit.For each of a plurality of revolutions of the medium, the methodcomprises injecting a plurality of different phase offsets into the readchannel to cause oversampling of the readback waveform at anoversampling rate higher than the sampling rate. The method alsocomprises measuring a metric or a phenomenon of disk drive operationthat requires sampling of the readback waveform at the oversamplingrate.

Some embodiments are directed to a method comprising obtaining, via aread channel of a disk drive, a readback waveform from a magneticrecording medium at a sampling rate of one sample per one written bit.For each of a plurality of revolutions of the medium, the methodcomprises injecting a plurality of different phase offsets into the readchannel. The method also comprises storing, in a buffer, an amplitude ofthe readback waveform for each of the different phase offsets. Themethod further comprises generating an oversampled readback waveformusing the amplitudes stored in the buffer.

Other embodiments are directed to an apparatus comprising a read channelconfigured to obtain an analog readback waveform from a magneticrecording medium of a disk drive at a sampling rate of one sample perone written bit. A buffer is coupled the read channel. Circuitry isconfigured to inject a plurality of different phase offsets into theread channel for each of a plurality of revolutions of the medium. Thecircuitry is also configured to store, in a buffer, an amplitude of thereadback waveform for each of the different phase offsets. The circuitryis further configured to generate an oversampled readback waveform usingthe amplitudes stored in the buffer.

The above summary is not intended to describe each disclosed embodimentor every implementation of the present disclosure. The Figures and thedetailed description below more particularly exemplify illustrativeembodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates components of a hard disk drive (HDD) including aread channel in which oversampling of a readback waveform can beimplemented according to various embodiments;

FIG. 2A illustrates a portion of the read channel shown in FIG. 1, andfurther shows components that, together with the read channel, areconfigured to implement readback waveform oversampling in accordancewith embodiments of the disclosure;

FIG. 2B illustrates representative data comprising a preamble and userdata in accordance with embodiments of the disclosure;

FIG. 3 shows a method of implementing readback waveform oversampling inaccordance with various embodiments;

FIG. 4 shows a method of implementing readback waveform oversampling inaccordance with various embodiments;

FIG. 5 is a representation of a readback waveform sampled at a samplingrate of one sample per one written bit during a first revolution of themagnetic recording medium in accordance with various embodiments;

FIG. 6 is a representation of readback waveform sampled after injectionof a +10% phase offset into the read channel in accordance with variousembodiments;

FIG. 7 is a representation of readback waveform sampled after injectionof a +40% phase offset into the read channel in accordance with variousembodiments;

FIG. 8 is a representation of readback waveform sampled after injectionof a −30% phase offset into the read channel in accordance with variousembodiments;

FIG. 9 shows an oversampled readback waveform in comparison to a nominalreadback waveform (no oversampling) in accordance with variousembodiments;

FIG. 10 shows a method of implementing readback waveform oversampling inaccordance with various embodiments;

FIG. 11 shows a method of implementing readback waveform oversampling inaccordance with various embodiments;

FIG. 12 illustrates an approach to generating an oversampled readbackwaveform by injecting one phase offset into the read channel for eachrevolution of the medium;

FIG. 13 illustrates an approach to generating an oversampled readbackwaveform by injecting a multiplicity of phase offsets into the readchannel for each revolution of the medium;

FIG. 14 shows a perspective view of a heat-assisted magnetic recording(HAMR) slider configuration according to some representativeembodiments; and

FIG. 15 shows a perspective view of a HAMR slider configurationaccording to other representative embodiments.

The figures are not necessarily to scale. Like numbers used in thefigures refer to like components. However, it will be understood thatthe use of a number to refer to a component in a given figure is notintended to limit the component in another figure labeled with the samenumber.

DETAILED DESCRIPTION

In the following description, reference is made to the accompanying setof drawings that form a part of the description hereof and in which areshown by way of illustration several specific embodiments. It is to beunderstood that other embodiments are contemplated and may be madewithout departing from the scope of the present disclosure. Thefollowing detailed description, therefore, is not to be taken in alimiting sense.

FIG. 1 illustrates components of a hard disk drive including a readchannel with which oversampling of a readback waveform can beimplemented according to various embodiments. The HDD includes arecording head 104 in proximity to a magnetic recording medium 102. Thehead 104 includes at least a reader and a writer. In some embodiments,the head 104 is configured for perpendicular magnetic recording (PMR).In other embodiments, the head 104 is configured for HAMR and, inaddition to a reader and a writer, includes a near-field transducer(NFT) proximate the writer, a laser source, and an optical waveguidethat optically couples laser light from the laser source to the NFT. Thereader (e.g., a magneto-resistive reader) of the head 104 senses themagnetic flux from the medium 102 and generates an analog readbackwaveform. The reader of the head 104 transmits a resistance readbackwaveform that is received by preamplifier 106, which converts theresistance waveform into a voltage waveform. The preamplifier 106provides the voltage readback waveform to a read channel 110, in whichembodiments of the disclosure can be implemented. A controller 101(e.g., a microprocessor, ASIC, or other general- or special-purposelogic circuitry) can be configured to coordinate operations of thecomponents shown in FIG. 1 and in other figures.

The read channel 110 is generally configured to perform a partialresponse maximum likelihood (PRML) approach to detecting and decodingdata read from the medium 102. Typical components of the read channel110 include a high pass filter (HPF) 111, a variable gain amplifier(VGA) 112, a low pass filter 114 (e.g., a Continuous Time Filter orCTF), an analog-to-digital converter (ADC) 116, a digital filter 118(e.g., a Finite Impulse Response or FIR filter), and a Viterbi detector122 coupled to a decoder 124. The HPF 111 receives the readbackwaveform, in the form of a time-varying voltage waveform, frompreamplifier 106, and high pass filters the readback waveform. Thefiltered readback waveform is received by the VGA 112, which produces anamplified readback waveform in accordance with the tolerances of ADC116, and transfers the readback waveform to the CTF 114. The filteredreadback waveform is sampled by ADC 116. The samples produced by the ADC116 are passed through the FIR filter 118 to fit the samples to thedesired channel response. These samples are then applied to the Viterbidetector 122 which generates encoded data that can be decoded by thedecoder 124 to complete the maximum likelihood detection process.

FIG. 2A illustrates a portion of the read channel 110 shown in FIG. 1.FIG. 2 also shows components that, together with the read channel 110,are configured to implement readback waveform oversampling in accordancewith embodiments of the disclosure. FIG. 2 shows a timing recovery block130 coupled to the ADC 116. To compensate for differences between thefrequency and phase at which data is written and read, timing recoverycan be implemented by the timing recovery block 130. For example, thetiming recovery block 130 can implement a method of timing recovery thatsynchronizes the sampling instances such that the frequency and phase atwhich the data is read corresponds to the phase and frequency at whichit was written. In order to recover the desired sampling instances,timing recovery is implemented by the timing recovery block 130 forprocessing both the servo sectors and the data sectors. Servo sectorscontain disk and track information written during manufacture that isutilized by the disk drive to determine the location of tracks andsectors on the magnetic recording disk. Data sectors are utilized tostore and retrieve user data. Thus, timing recovery performance affectsboth servo performance (i.e., the operation of the servo to position theread/write head) and the performance of the data processing.

The read channel 110 is configured to obtain an analog readback waveformfrom a magnetic recording medium of a disk drive at a sampling rate ofone sample per one written bit. The timing recovery block 130 isconfigured to recover the written-in phase of a readback waveform. Forexample, and with reference to FIG. 2B, representative data 202 is shownto include a preamble 204 and user data 208. Among other information,the preamble 204 includes Zero Phase Start (ZPS) and other adaptationfeatures 206 which can be used by the timing recovery block 130 toprovide an estimate of the written-in phase at the beginning of thepreamble 204. A phase injection block 132 is coupled to the timingrecovery block 130. The timing recovery block 130 is configured toreceive phase offset values generated by the phase injection block 132.The phase offset values produced by the phase injection block 132represent phase offsets to the readback phase (nominal phase) recoveredby the timing recovery block 130. The phase offsets can be positiveand/or negative phase offsets to the written-in phase.

Injecting phase offsets into the read channel 110 via the timingrecovery block 130 and the ADC 116 effectively changes the locationwhere the ADC 116 is sampling the readback waveform. For example, thephase injection block 132 can inject a phase offset relative to thereadback phase from −50% to +50% of the bit cell in a predeterminednumber of increments. In some embodiments, the predetermined number ofincrements can range between 2 and 32, which corresponds to anoversampling factor ranging between 2 and 32. For example, injecting aphase offset into the read channel 110 from −50% to +50% of the bit cellin 32 increments allows for a data pattern to be readback at plus orminus 50% of the nominal location, which corresponds to oversampling ofthe readback waveform by a factor of 32. A sample buffer 134 is coupledto the ADC 116. Sampling the readback waveform by the ADC 116 at anominal phase offset and a multiplicity of different phase offsetsinjected into the read channel 110 results in the generation of anoversampled readback waveform in the sample buffer 134.

FIG. 3 shows a method of implementing readback waveform oversampling inaccordance with various embodiments. The method shown in FIG. 3 involvesobtaining 302, via a read channel, a readback waveform from a magneticrecording medium of a disk drive at a sampling rate of one sample perone written bit. For each of a plurality of revolutions of the medium,the method involves injecting 304 one of a plurality of different phaseoffsets into the read channel. The method involves storing 306, in abuffer, an amplitude of the readback waveform for a nominal phase offsetand each of the different phase offsets. The method further involvesgenerating 308 an oversampled readback waveform using the amplitudesstored in the buffer.

FIG. 4 shows a method of implementing readback waveform oversampling inaccordance with other embodiments. The method shown in FIG. 4 involvesobtaining 402, via a read channel, a readback waveform from a magneticrecording medium of a disk drive at a sampling rate of one sample perone written bit. For each of a plurality of revolutions of the medium,the method involves injecting 404 one of a plurality of different phaseoffsets into the read channel to cause oversampling of the readbackwaveform at an oversampling rate higher than the sampling rate. Themethod further involves measuring 406, by a processor of the disk drive,a metric or a phenomenon of disk drive operation that requires samplingof the readback waveform at the oversampling rate.

For example, an oversampled readback waveform generated from readingPRBS (pseudo-random binary sequence) patterns can be used to measurevarious metrics of disk drive operation, such as ensemble waveformsignal-to-noise ratio (EWSNR), channel bit density (CBD), and dibitresponse. Measuring EWSNR, for example, typically requires oversamplingof the readback waveform by a factor ranging between 2 and 8. Variousphenomena of HAMR disk drive operation can be measured using anoversampled readback waveform, such as laser induced writer protrusion(LIWP), frequency mode hops, and a thermal gradient (TG). Measuring modehops and a thermal gradient, for example, typically requiresoversampling of the readback waveform by a factor of at least about 10.

In some embodiments, mode hops can be measured by reading many PRBSpatterns written within a sector, and aligning them against each otherto determine how much each has shifted relative to others and theirexpected location. This allows for increased resolution of the size andlocation of mode hops, and a better estimate of the number of mode hops.According to some embodiments, the thermal gradient can be measured byabruptly changing the laser power (ΔA) in the middle of a data sector(or a data wedge), giving rise to downtrack bit shifts (δ). The thermalgradient, TG, can be approximated by:

${TG} = {\left( {T_{Write} - T_{Ambient}} \right)*\frac{\Delta\; A}{\delta}}$where, T_(Write) is the temperature at which data is written, andT_(Ambient) is the ambient temperature of the media. The dibit responsecan be measured using an oversampled readback waveform in the mannerdescribed in commonly-owned U.S. Pat. No. 9,842,621, which isincorporated herein by reference. Mode hops can be measured using anoversampled readback waveform in the manner described in commonly-ownedU.S. Pat. No. 9,818,447, which is incorporated herein by reference.Channel bit density can be measured using an oversampled readbackwaveform in the manner described in commonly-owned U.S. Pat. No.9,947,362, which is incorporated herein by reference. EWSNR includes twonoise sources: transition noise (also called jitter) and remanencenoise. Transition noise refers to the SNR from writing a transition. Thevariation in writing the same location, for example, representstransition noise. This variation can be detected as a deviation of theSNR when writing a transition relative to a threshold. Remanence noiserepresents the variation in signal strength during a long mark, wellaway from a transition. Remanence noise can be detected as a deviationof the SNR when writing a long mark relative to a threshold.

FIGS. 5-8 facilitate an understanding of readback waveform oversamplingin accordance with various embodiments. It is understood that thespecific phase offsets and number of phase offsets associated with FIGS.5-8 are provided for purposes of illustration and not of limitation.FIGS. 5-8 show graphs with amplitude along the y-axis and bit numberalong the x-axis. FIG. 5 is a representation of a readback waveformsampled at a sampling rate of one sample per one written bit during afirst revolution of the magnetic recording medium. The amplitude of eachsample acquired during a first revolution of the medium at a nominalphase is shown as a solid dot in FIG. 5. During the first revolution ofthe medium, the amplitude of each sample is stored in a buffer (e.g., asample buffer).

At the completion of the first revolution of the medium, and withreference to FIG. 6, a +10% phase offset is injected into the readchannel followed by sampling of the readback waveform during a secondrevolution of the medium. The amplitude of each sample acquired duringthe second revolution of the medium at a +10% phase offset is shown as asquare. It can be seen in FIG. 6 that each sample denoted by a square isshifted in a positive direction along the x-axis by 10% of the distancebetween adjacent bits. As was discussed previously, injection of thephase offset changes the location where the channel is sampling thereadback waveform. During the second revolution of the medium, theamplitude of each sample is stored in the buffer. The buffer nowcontains the amplitudes of readback waveform samples for the nominalphase offset (revolution 1) and the +10% phase offset (revolution 2).

At the completion of the second revolution of the medium, and withreference to FIG. 7, a +40% phase offset is injected into the readchannel followed by sampling of the readback waveform during a thirdrevolution of the medium. The amplitude of each sample acquired duringthe third revolution of the medium at a +40% phase offset is shown as acircle. It can be seen in FIG. 7 that each sample denoted by a circle isshifted in a positive direction along the x-axis by 40% of the distancebetween adjacent bits. During the third revolution of the medium, theamplitude of each sample is stored in the buffer. The buffer nowcontains the amplitudes of readback waveform samples for the nominalphase offset (revolution 1), the +10% phase offset (revolution 2), andthe +40% phase offset (revolution 3).

At the completion of the third revolution of the medium, and withreference to FIG. 8, a −30% phase offset is injected into the readchannel followed by sampling of the readback waveform during a fourthrevolution of the medium. The amplitude of each sample acquired duringthe fourth revolution of the medium at a −30% phase offset is shown as atriangle. It can be seen in FIG. 8 that each sample denoted by atriangle is shifted in a negative direction along the x-axis by 30% ofthe distance between adjacent bits. During the fourth revolution of themedium, the amplitude of each sample is stored in the buffer. The buffernow contains the amplitudes of readback waveform samples for the nominalphase offset (revolution 1), the +10% phase offset (revolution 2), the+40% phase offset (revolution 3), and the −30% phase offset (revolution4).

It is understood that the process of injecting a phase offset andsampling the readback waveform for each of a number of revolutions ofthe medium can continue until a desired rate of oversampling is achieved(e.g., 2×, 8×, 10×, 20×, 32×). FIG. 9 shows an oversampled readbackwaveform 902 in comparison to a nominal readback waveform 904 (nooversampling). The oversampled readback waveform 902 was generated inthe manner discussed above and has an oversampling factor of 32 (e.g.,32 samples between adjacent bit cells). In contrast, the nominalreadback waveform 904 was sampled at the nominal phase offset with nooversampling (e.g., 1 sample per bit cell). It can be seen that theoversampled readback waveform 902 has a significantly greater resolutionthan the nominal readback waveform 904. It is noted that a digital scopeused in the factory to evaluate readback waveforms typically obtains 20samples per written bit, whereas the oversampled readback waveform 902obtains 32 samples per written bit, thus providing better resolutionthan the digital scope.

According to the embodiments discussed hereinabove, generating anoversampled readback waveform involves injection of a single phaseoffset into the read channel for each revolution of the magneticrecording medium. In accordance with other embodiments, generating anoversampled readback waveform involves injection of a multiplicity ofphase offsets into the read channel for each revolution of the magneticrecording medium. Injecting a multiplicity of phase offsets into theread channel for each revolution of the medium advantageously results ina concomitant reduction in the number of reads and amount of timerequired to generate an oversampled readback waveform.

FIG. 10 shows a method of implementing readback waveform oversampling inaccordance with various embodiments. The method shown in FIG. 10involves obtaining 1002, via a read channel, a readback waveform from amagnetic recording medium of a disk drive at a sampling rate of onesample per one written bit. For each of a plurality of revolutions ofthe medium, the method involves injecting 1004 a plurality of differentphase offsets into the read channel. The method involves storing 1006,in a buffer, a waveform defined by an amplitude of sub-waveforms eachassociated with one of the different phase offsets. The method furtherinvolves generating 1008 an oversampled readback waveform using theamplitudes stored in the buffer. The oversampled readback waveform is awaveform formed from the plurality of sub-waveforms each associated withone of the different phase offsets.

FIG. 11 shows a method of implementing readback waveform oversampling inaccordance with other embodiments. The method shown in FIG. 11 involvesobtaining 1102, via a read channel, a readback waveform from a magneticrecording medium of a disk drive at a sampling rate of one sample perone written bit. For each of a plurality of revolutions of the medium,the method involves injecting 1104 a plurality of different phaseoffsets into the read channel to cause oversampling of the readbackwaveform at an oversampling rate higher than the sampling rate. Themethod further involves measuring 1106, by a processor of the diskdrive, a metric or a phenomenon of disk drive operation that requiressampling of the readback waveform at the oversampling rate.

For purposes of illustrating the advantages of injecting a multiplicityof phase offsets into the read channel for each revolution of themedium, reference is made to FIGS. 12 and 13. FIG. 12 illustrates anapproach to generating an oversampled readback waveform by injecting onephase offset into the read channel for each revolution of the medium.FIG. 13 illustrates an approach to generating an oversampled readbackwaveform by injecting a multiplicity of phase offsets into the readchannel for each revolution of the medium. In the graphs shown in FIGS.12 and 13, the y-axis is the phase offset given in percentage, and thex-axis is the length into a data wedge given in percentage.

FIG. 12 shows injection of a single phase offset for each read of thedata wedge. As shown, during a first revolution of the medium, a firstread 1201 produces a single waveform that is sampled at a locationassociated with a phase offset of +18%. During a second revolution ofthe medium, a second read 1202 produces a single waveform that issampled at a location associated with a phase offset of +35%. During athird revolution of the medium, a third read 1203 produces a singlewaveform that is sampled at a location associated with a phase offset of+50%. During a fourth revolution of the medium, a fourth read 1204produces a single waveform that is sampled at a location associated witha phase offset of −18%. During a fifth revolution of the medium, a fifthread 1205 produces a single waveform that is sampled at a locationassociated with a phase offset of −35%. During a sixth revolution of themedium, a sixth read 1206 produces a single waveform that is sampled ata location associated with a phase offset of −50%. It can be seen inFIG. 12 that six reads (and six revolutions of the medium) are requiredto generate a readback waveform that is oversampled by a factor of 6(6×).

FIG. 13 shows injection of a multiplicity of phase offsets for each readof the data wedge. As shown, during a first revolution of the medium, afirst read 1301 produces three different waveforms (Waveforms 1-3), alsoreferred to as sub-waveforms, that are sampled at a three differentlocations in the data wedge, each associated with a different phaseoffset. More particularly, the first read 1301 produces Waveform 1 whichis sampled at a first location in the data wedge (between 0 and 35%)associated with a phase offset of +18%. The first read 1301 alsoproduces Waveform 2 which is sampled at a second location in the datawedge (between 35 and 65%) associated with a phase offset of +35%. Thefirst read 1301 further produces Waveform 3 which is sampled at a thirdlocation in the data wedge (between 65 and 100%) associated with a phaseoffset of +50%.

During a second revolution of the medium, a second read 1302 producesthree different waveforms (Waveforms 4-6) that are sampled at a threedifferent locations in the data wedge, each associated with a differentphase offset. More particularly, the second read 1302 produces Waveform4 which is sampled at a first location in the data wedge (between 0 and35%) associated with a phase offset of −18%. The second read 1302 alsoproduces Waveform 5 which is sampled at a second location in the datawedge (between 35 and 65%) associated with a phase offset of −35%. Thesecond read 1302 further produces Waveform 6 which is sampled at a thirdlocation in the data wedge (between 65 and 100%) associated with a phaseoffset of −50%. It can be seen in FIG. 13 that two reads (and tworevolutions of the medium) are required to generate a readback waveformthat is oversampled by a factor of 6×. As was discussed previously,injecting a multiplicity of phase offsets into the read channel for eachrevolution of the medium advantageously results in a concomitantreduction in the number of reads and amount of time required to generatean oversampled readback waveform.

Readback waveform oversampling according to the present disclosure canbe implemented in any type of hard disk drive (HDD), such as thoseconfigured for PMR and those configured for HAMR. In heat-assistedmagnetic recording devices, also sometimes referred to asthermal-assisted magnetic recording (TAMR) devices or energy assistedmagnetic recording (EAMR) devices, a magnetic recording medium (e.g.,hard drive disk) is able to overcome superparamagnetic effects thatlimit the areal data density of typical magnetic media. In a HAMR diskdrive, information bits are recorded on a storage layer at elevatedtemperatures. The heated area in the storage layer determines the databit dimension, and linear recording density is determined by themagnetic transitions between the data bits.

In order to achieve desired data density, a HAMR recording head (e.g.,slider) includes optical components that direct light from a laser tothe recording media. In heat-assisted magnetic recording, a mediahotspot (thermal hotspot) is created using the laser. This thermalhotspot generally needs to be smaller than a half-wavelength of lightavailable from current light sources (e.g., laser diodes). Due to whatis known as the diffraction limit, optical components cannot focus thelight at this scale. One way to achieve tiny confined hot spots is touse an optical near-field transducer (NFT), such as a plasmonic opticalantenna. The NFT is designed to support local surface-plasmons at adesigned light wavelength. At resonance, a high electric field surroundsthe NFT due to the collective oscillation of electrons in the metal.Part of the field impinges on the magnetic recording medium, raising thetemperature of the medium locally for recording. During recording, awrite element (e.g., write pole) applies a magnetic field to the heatedportion (thermal hotspot) of the medium. The heat lowers the magneticcoercivity of the medium, allowing the applied field to change themagnetic orientation of the heated portion. The magnetic orientation ofthe heated portion determines whether a one or a zero is recorded. Byvarying the magnetic field applied to the magnetic recording mediumwhile it is moving, data is encoded onto the medium.

A HAMR disk drive, for example, uses a laser diode to heat the magneticrecording medium to aid in the recording process. FIGS. 14 and 15 showperspective views of HAMR slider configurations according torepresentative embodiments. For simplicity, like reference numbers areused in FIGS. 14 and 15. In FIG. 14, a slider 1400 has a laser-in-slider(LIS) configuration. In this configuration, the slider 1400 includes aslider body 1401 having an edge-emitting laser diode 1402 integratedinto a trailing edge surface 1404 of the slider body 1401. In thisexample, the laser diode 1402 is disposed within a cavity formed in thetrailing edge surface 1404. The laser diode 1402 is proximate to a HAMRread/write element 1406, which has one edge on an air bearing surface1408 of the slider 1400. The air bearing surface 1408 faces and is heldproximate to a moving magnetic media surface (not shown) during deviceoperation.

While here the read/write element 1406 is shown as a single unit, thistype of device may have a physically and electrically separate readelement (e.g., magnetoresistive stack) and write element (e.g., a writecoil and pole) that are located in the same general region of the slider1400. The separate read and write portion of the read/write element 1406may be separately controlled (e.g., having different signal lines,different head-to-media spacing control elements, etc.), although mayshare some common elements (e.g., common signal return path). It will beunderstood that the concepts described relative to the read/writeelement 1406 may be applicable to individual read or write portionsthereof, and may be also applicable where multiple ones of the readwrite portions are used, e.g., two or more read elements, two or morewrite elements, etc.

The laser diode 1402 provides electromagnetic energy to heat the mediasurface at a point near to the read/write element 1406. Optical pathcomponents, such as a waveguide 1410, are formed integrally within theslider 1400 to deliver light from the laser diode 1402 to the media. Inparticular, a local waveguide and NFT 1412 may be located proximate theread/write element 1406 to provide local heating of the media duringwrite operations.

In FIG. 15, a laser-on-slider (LOS) configuration 1420 is illustrated.This example includes a laser diode 1422 that is mounted on a topsurface of a slider body 1421. The laser diode 1422 is coupled to anoptical path of the slider body 1421 that includes, among other things,an optical path 1424 (e.g., a straight waveguide). In thisconfiguration, the laser diode 1422 may also be edge-emitting, such thatthe light is emitted from the laser diode 1422. In order to direct thelight towards the air bearing surface 1408, the laser diode 1422 (orother component) may include optical path elements such as a mirror (notshown) that redirects the light emitted from the laser diode 1422towards the air bearing surface 1408. In other configurations, anedge-emitting, top-mounted laser diode may be oriented so that the lightemitted directly downwards toward the air bearing surface 1408. This mayinvolve placing the laser diode 1422 on a submount (not shown) on thetop of the slider body 1421, the submount orienting the laser output inthe desired direction. While other components shown in FIG. 15, such asthe NFT 1412 and read/write element 1406, are referenced using the samenumbers as FIG. 14, the physical configuration of these and othercomponents may differ in the different slider arrangements, e.g., due tothe differences in optical coupling pathways, materials, laser power,etc.

Systems, devices or methods disclosed herein may include one or more ofthe features structures, methods, or combination thereof describedherein. For example, a device or method may be implemented to includeone or more of the features and/or processes above. It is intended thatsuch device or method need not include all of the features and/orprocesses described herein, but may be implemented to include selectedfeatures and/or processes that provide useful structures and/orfunctionality.

Various modifications and additions can be made to the disclosedembodiments discussed above. Accordingly, the scope of the presentdisclosure should not be limited by the particular embodiments describedabove, but should be defined only by the claims set forth below andequivalents thereof.

What is claimed is:
 1. A method, comprising: obtaining, via a readchannel of a disk drive, a readback waveform from a magnetic recordingmedium at a sampling rate of one sample per one written bit; for each ofa plurality of revolutions of the medium, injecting a plurality ofdifferent phase offsets into the read channel; storing, in a buffer, awaveform defined by an amplitude of sub-waveforms each associated withone of the different phase offsets; and generating an oversampledreadback waveform using the amplitudes stored in the buffer.
 2. Themethod of claim 1, wherein the phase offsets range from about −50% toabout +50% relative to a nominal phase of the readback waveform.
 3. Themethod of claim 1, wherein: obtaining the readback waveform comprisesrecovering a written-in phase from a preamble of the readback waveform;and injecting the phase offsets comprises injecting positive phaseoffsets and negative phase offsets into the read channel relative to thewritten-in phase.
 4. The method of claim 1, wherein the readbackwaveform is oversampled by a factor ranging from 2 to
 32. 5. Anapparatus, comprising: a read channel configured to obtain an analogreadback waveform from a magnetic recording medium of a disk drive at asampling rate of one sample per one written bit; a buffer coupled theread channel; and circuitry configured to: for each of a plurality ofrevolutions of the medium, inject a plurality of different phase offsetsinto the read channel; store, in a buffer, an amplitude of the readbackwaveform for each of the different phase offsets; and generate anoversampled readback waveform using the amplitudes stored in the buffer.6. The apparatus of claim 5, wherein the circuitry comprises: a timingrecovery block coupled to an analog-digital-converter (ADC) of the readchannel; and a phase injection block coupled to the timing recoveryblock; wherein the timing recover block is configured to inject thedifferent phase offsets into the ADC in response to phase offset valuesreceived from the phase injection block.
 7. The apparatus of claim 5,wherein the phase offsets range from about −50% to about +50% relativeto a nominal phase of the readback waveform.
 8. The apparatus of claim5, wherein the timing recovery block is configured to: recover awritten-in phase from a preamble of the readback waveform; and injectpositive phase offsets and negative phase offsets into the ADC relativeto the written-in phase.
 9. The apparatus of claim 5, wherein thereadback waveform is oversampled by a factor ranging from 2 to
 32. 10.The apparatus of claim 5, wherein the circuitry comprises a processorconfigured to measure a metric or a phenomenon of disk drive operationthat requires sampling of the readback waveform at the oversamplingrate.
 11. The apparatus of claim 5, wherein the disk drive is aheat-assisted magnetic recording (HAMR) disk drive.